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Evaluation of Digital Elevation Models created from different satellite images
DEM generation process
GCPs and stereo GCPs are collected from reference maps in order to reference the images to ground. Further, TPs (Tie points) are collected to improve matching between the two stereo pairs. The following table gives the number of GCPs and Tie Points used to create a DEM:
Once registered to the same ground area, any along track (in case of SPOT, IRS and Radarsat images) or across track (in case of ASTER images) positional differences are assumed to be due to parallax, which results due to relief. Measured parallax differences on a pixel-by-pixel basis are converted to absolute elevations using trigonometric functions and the orbital data (orbital position, altitude, attitude and the scene center). The computation relies on the inherent parallax between stereo images. An automated image correlation algorithm [1] is used to derive elevations from the parallax, by a set of well located GCPs and tie points (TPs). The image matching system operates on a reference and a search window. For each position in the search window, a match value is computed from gray level values in the reference window. The match value is computed with the mean normalized cross-correlation coefficient and the sum of mean normalized absolute difference [2]. The correlation window size varies from low resolution (8 pixels) to 32 pixels at the full resolution. Elevation points are extracted at every pixel for the complete stereo pair. The 3-D intersection is performed using the above computed geometric model [3] to convert the pixel coordinates in both images determined in the image matching of the stereo pair to the three dimensional data [4]. The output elevations are not computed for the pixels where the image matching fails to find the corresponding pixel in the reference image, resulting into some failure areas. In case of small and scattered failures the software does interpolate and compute most probable values for them. The DEM thus generated is in raw state and does not contain geo-referencing information. So, the DEM needs to be georeferenced. Workflow 
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